floss: Handle callbacks from btgatt_client_callbacks_t

This adds handlers for:
* disconnect_cb
* search_complete_cb
* register_for_notification_cb
* notify_cb

Bug: 193685325
Tag: #floss
Test: Build floss on Linux and AOSP

Change-Id: I093f0b3f4394d9cdc641ae74a15896dd479eb24f